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Geochemical Exploration – From Test Tube to Mass Spectrometer

Ray Lett, BC Geological Survey

Abstract I’ve been fortunate to have watched the development geochemical exploration techniques from a time when they were regarded with mild amusement by the mining community to the present day where geochemistry is considered a key discipline contributing to the search for new mineral deposits. In fact one of the pioneers of exploration geochemistry, Dr. Harry Warren, started his research at the UBC Geology Department in the 1950’s. Even though our geochemical tool box has grown over the past 50 years to include a wide range of instrumental analytical methods such as inductively coupled plasma mass spectrometry that can reliably determine over half the elements in the periodic table and the use of sophisticated computers to display and process multi-element data the basics of any successful geochemical survey remain the same. These are selecting the optimum sample medium, sample preparation method and analytical technique that will best identify a source of mineralization and correctly interpreting survey data.

Introduction I still have in my library a 1962 edition of “Geochemistry in Mineral Exploration” by H.E. Hawkes and J.S. Webb, now somewhat battered and water stained. Hawkes and Webb clearly define geochemical prospecting to be “any method of mineral exploration based on systematic measurement of one or more chemical properties of a naturally occurring material”. The authors were fortunate that in the 1960’s the range of chemical properties that could be routinely measured was still relatively small. Modern analytical techniques allow precise and accurate determination of elements in geochemical samples from more than half of the periodic table to levels reaching parts per trillion concentrations. A typical geochemical survey can now generate many thousand field and analytical measurements so that the exploration geologist or geochemist can face a daunting task when attempting to interpret the survey results. A better understanding of the design, sampling, sample preparation and sample analysis stages of a survey can help simplify the compilation, interpretation and reporting of the results. Clearly only a few topics can be covered in the time available. Therefore the focus will be on the basics to try and answer questions that are often asked before and during a survey i.e. • What to sample, where and how? • How to analyse the samples and for what? • What do the numbers mean? Among topics covered in this talk are: • Basics of geochemical dispersion • Drainage sampling – Streams, lakes, moss mat sediments, heavy minerals

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• Some tools for Interpretation

The Basics – Definitions - Dispersion An anomaly is any high or low element variation not explained by a natural geochemical variation. Key to interpreting survey results is determining what is the natural variation or geochemical background for different elements. The upper limit of natural background is termed threshold above which a value is considered to be anomalous. Closely linked to establishing threshold is selecting sampling and analytical techniques that will give maximum anomaly contrast (i.e. peak value to threshold ratio or signal to noise ratio). Recognizing the dominant mechanism governing element migration in an area can help better design a survey so that the best anomaly contrast will be obtained. Two major processes responsible for element migration and concentration are:

• Physical Dispersion: This process is purely mechanical; products mix, but do not separate into specialized fractions except sorting. Anomalies form at sites of mineral concentration (e.g. heavy mineral capture in high-energy streams, mineral capture by moss, transport of rock fragments in till).

• Chemical (hydromorphic) Dispersion: This process produces fractions with

widely differing chemical composition (e.g. Cu2+, CuS, Cu-organic matter). Chemical weathering processes (e.g. oxidation of pyrite, solution of calcium sulphate, solution of calcium carbonate) are dominant releasing elements from rock-forming minerals. Anomalies form through such processes as precipitation reactions in response to changing pH and redox potential, surface adsorption (e.g. zinc to manganese oxide) controlled by pH and the formation of natural organic complexes (e.g. copper, uranium).

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The basic dispersion process Thermodynamic pH-redox potential (Eh) diagrams can help predict the chemical behaviour of an element in the near-surface weathering environment. For example, in alkaline, reduced water logged soil geochemically mobile elements (e.g. copper, iron) might accumulate as secondary mineral sulphides. More oxidizing and acidic surface waters increase mobility of elements in cation form (e.g. copper , zinc) but will reduce migration of elements as anions (e.g. molybdenum). Predicting the chemical of physical form of element can help decide on an analytical method (e.g. a partial extraction technique) to increase anomaly contrast and the ability of the survey to detect mineralization.

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Natural pH – Eh fields (from Garrels & Christ, 1963) The pH control on element mobility can be illustrated by the dispersion of lead and zinc in sediment downstream from SEDEX style Pb-Ag-Ba-Zn mineralization at the Bear occurrence in NE BC. The large lead variation in the sediment suggests physical weathering and transport of lead as galena in the drainage whereas the zinc variations correspond to water pH changes. The higher pH is probably due to the presence of carbonate horizons whereas the low pH reflects oxidizing sulphides in black shale. High (e.g. percent level) concentrations of metals such as copper and lead can accumulate in wetlands and seepage areas where organic sediment absorbs metal or secondary minerals precipitate in the high sulphide environment. This will result in an abnormally high metal background in soil and sediment.

Lead and zinc in stream sediments – Bear occurrence NE BC

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Generalized dispersion model Some of the dispersion processes responsible for formation of sediment and soil anomalies are summarized in a conceptual, three-dimensional model. This shows the variation of a metal such as copper down a typical soil profile on well drained and poorly drained soils modified by glacial transport of the parent overburden.

The Survey Plan Information sources that can be used to plan geochemical surveys include: • Geology & mineral deposit models • Mineral deposit database (MINFILE) • Regional Geochemical Surveys • Mineral deposit models • Orientation surveys • Regional Geophysics • Assessment files Factors that should be considered in designing a survey are Budget; Target size (e.g. VMS versus porphyry); Survey scale (e.g. regional or detailed) and existing geochemical data for the proposed area.

When in doubt about survey design - Do an orientation survey first!!

Common Sample Types

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Among the more common sample media used for geochemical sampling are stream and lake sediment, ground and surface water, glacial sediment (till), soil, rock, vegetation and soil gas. Drainage Surveys. Drainage surveys are based on the concept that fluvial and chemical processes carry metals and minerals from within a drainage catchment (watershed) to a site of mineral (& metal) accumulation (an anomaly). The source of the minerals can be weathered bedrock, glacially transported sediment (e.g. till) or previous fluvial deposits. Also, the source area is well defined by the limits of the drainage basin. Elements will be transported by a combination of physical and chemical fluvial processes, the dominant depending on climate & the characteristics of the stream catchment basin (e.g. size, topographic relief). Anomalous metal (or mineral) content of sediment decays down stream forming a dispersion train. The exponential decay curve reflects dilution from non-mineralized material. Catching the dispersion train that is correctly identifying just what is anomalous is a key to detecting a mineralized source. Determining just what constitutes an anomaly for different elements relies on estimating geochemical background. This will depend on bedrock and surficial chemistry and the characteristics of the near surface environment. For example, shale can have a high Zn

background. Organic-rich lake sediments can also have a high background of more mobile metals such as copper and molybdenum. This cartoon shows the relationship between a source (a geochemical soil anomaly), the catchment basin, anomaly decay curve and a feasible sediment sample point where it is hoped that evidence of the soil anomaly can be detected in the stream sediment geochemistry. In reality the catchment

basin size will be larger than shown in the cartoon and will be defined by the topographic limits of the watershed. The dispersion train can be quite short for chemically less mobile elements (e.g. gold, platinum, tin), but longer for more mobile elements such as zinc. An anomaly can reflect a change in water chemistry such as an increase in water pH due to solution of a limestone unit crossing the creek. At that point there could be a sharp increase in the content of metal (e.g. copper) in sediment. Other factors responsible for metal (and mineral) accumulation giving rise to an anomaly are a change in stream gradient (physical process), bank collapse (landslides), decoupling of the channel from the valley side (increased exotic sediment in the stream) and the presence of manganese oxide coatings on boulders (chemical – adsorption of metals).

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This shows a typical reconnaissance scale stream sediment-water survey in progress. The sediment is being collected from the finer textured material on the point bar. This fluvial sediment is most suitable for detecting more geochemically mobile elements such as molybdenum, copper, arsenic, nickel and zinc, but is less ideal for detecting more physically transported elements such as barium, gold, tin, tungsten & lead

because of larger sampling variability associated with more dense, detrital minerals (i.e. “nugget” effect). However, for large regional surveys a compromise is needed between speed and ease of collection against the ability of the survey to detect anomalous levels of all of the elements that may be present in the source catchment area. During this reconnaissance scale stream sediment-water survey in the Bowser basin area on North-western British Columbia both sediment and stream water samples were taken at an average density of 1 sample/ 13km2. About 0.5 to 1 Kg of sediment is collected in a Kraft high wet strength bag or a polyester weave bag. The water is collected in a 250 ml Nalgene bottle. An important precaution is that samplers must not wear jewelry (e.g. gold rings) to avoid sample contamination during collection. (Photo credit - Wayne Jackaman).

Any survey has to be designed so that sample locations will avoid obvious contamination from industrial, human and forestry sources. In this cartoon a sediment sample site is ideally located on a lateral bar and more than 60 metres upstream from a bridge, a collection of rotting oil drums, a tributary drainage and in a riparian zone of trees along the drainage channel. The sampler selects the final site.

The BC Regional Geochemical Survey (RGS) As part of the National Geochemical Reconnaissance (NGR) program reconnaissance-scale drainage sediment and water surveys have been carried out since 1976 including lake sediment-water, till & heavy mineral sampling. The BC RGS Aims are to: • Identify areas of high mineral potential;

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• Produce baseline geochemical data (e.g. for environmental monitoring); • Test new geochemical methods for finding new mineral deposits; • Help train a new generation of exploration geochemists and; • Helps industry find new exploration targets (although the reconnaissance scale

sample density may be too low to always be successful). The map below shows the coverage of the different types of geochemical survey carried out by government and non-government (e.g. Geoscience BC) agencies in British Columbia. The slide identifies more detailed, higher density surveys undertaken to study specific problems or evaluate specific areas.

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Contoured arsenic in about 53,000 drainage sediments with the location of gold occurrences

Recording Sample Information The Geological Survey of Canada field information forms have options for collecting field information listed in the different categories so that the sampler just “ticks the box” for the variable in each category. One of the categories is to identify field duplicate samples recognising that a good quality control program starts in the field. The quality control relies on inserting standards and duplicates before analysis. In the field duplicate samples, generally taken 2-3 metres separation from the same sediment types will be inserted randomly whereas replicate analytical samples and standards are inserted in blocks of 20 samples collected after sample preparation, but before analysis. In each batch of 20 samples there is a reserved position for insertion of analytical replicates (generally a split on the first field duplicate sample) and for a standard reference material. Different forms are used for lake sediment and soil surveys but in all cases quality control is recognized. After collection, drainage sample drying is important not only to make the wet samples less susceptible to damage during shipping but also to allow a check of samples with insufficient material. Bags are identified by National Topographic System (NTS) map sheet number and a 4 digit sample number. All bags and bottles (including those for the “reserved sites” in the blocks of 20) are numbered before the survey starts. If several crews are sampling at the same time pre-numbered sets grouped between 1001 to 1500, 3001 – 3500 and 7001 – 7500.

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Sample Analysis In general the factors to be considered when deciding on the analytical methods to be used are: • Preparation – the optimum size or density fraction for analysis • Analysis – “total” versus “partial extraction method • Quality control monitoring - how reliable are those numbers?

For example, stream sediment, moss sediment and lake sediment samples are dried at 35 to 40 oC before preparation. Stream sediments and moss sediments are disaggregated and sieved to the < 0.177 mm (- 80 mesh) fraction through a nylon or stainless steel screen in the laboratory before inserting quality control standards and duplicate splits of the sieved samples into each batch of 20 samples. Lake sediments are milled in a ceramic mill to – 150 mesh before analysis. Routine RGS sediment analysis involves: • Determination of 33 elements (including Au, U, rare earth elements) by instrumental

neutron activation (INAA) • Determination of up to 56 elements (including Cu, Pb, Zn, S) by aqua regia

digestion-inductively coupled mass spectrometry (ICPMS). • Determination of Loss on ignition (LOI), fluorine and for some surveys, tin. • Analysis of Water sampled for pH, F, U and for some surveys, major elements, trace

elements by ICPMS.

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Other Common Drainage Sample Types Regional survey where samples are collected between 1 /10 km2 and 1 /15 km2 will outline belts where the geology has elevated content of certain elements. However, the sample density is often too low to detect individual deposits. The routine survey can be improved by adjusting sample density so that the survey will better detect mineralization or by using a different sample types. Lake sediments are an obvious example to deal with areas where there are few streams, but many lakes. Moss mat sediment collection was introduced in the early 1990’s to solve the problem of sampling fast flowing mountain streams on Vancouver Island where there was depletion silt-sand sized sediment in the stream bed-load leaving mainly gravel and boulders. Much of British Columbia is mountainous and has well-developed drainage systems. This is ideal topography for stream sediment surveys and hence much of the province has been covered. However, a large part of the British Columbia interior is flat and is covered by glacial deposits and recent plateau basalts so that there are few, well-defined drainages, but numerous lakes. The obvious strategy for regional geochemical surveys would be to use lakes rather than streams.

This cartoon shows a typical mineral source from which mineralized rock material has been glacially transported into the lake. In reality, the lake catchment area would be much larger that actually shown in the cartoon.

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This cartoon shows the various pathways along which metals can be dispersed into a lake basin. A stream dispersion train is one obvious source and ground water another source. In both cases metals will be captured by the organic-rich, gelatinous center lake sediment. An example of this type of sediment is shown in the insert. Clastic dispersion gives rise to a near shore lake sediment anomaly.

Here’s a view of emptying a “Hornbrook” type sediment sampler during routine Geoscience BC funded helicopter-supported lake sediment survey. The sampler is about 40 cm long & 3 cm diameter and is weighted close to the lower end to increase penetration into the sediment. It has a one-way flap valve in the tube to retain the sediment. The sampler is attached to a nylon rope (marked at meter and 0.5 metre units) and simply dropped from the deck when the helicopter is stationary on the lake. Generally there is sufficient momentum for the sampler to penetrate 20 to 30 cm into the sediment. The sampler is pulled up by hand and the material transferred to a sample

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bag with a scoop. This slide shows transferring material from the sampler to a bag. This Geoscience BC funded survey was carried out recently in central British Columbia by Wayne Jackaman. Photo credit - Wayne Jackaman.

The map above shows the distribution of copper in center lake sediment. The eastern part of the survey area is underlain by predominantly Mesozoic volcanic island arc rocks that host porphyry Cu-Mo (e.g. Gibraltar) and porphyry Cu-Au (e.g. Mount Polley) deposits. West of Williams Lake area predominantly Eocene plateau volcanic and Mesozoic sedimentary (Cache Creek Group) rocks covered by extensive glacial sediments. Note that there is a strong cluster of anomalies (> 59 ppm) in the SE part of the survey area. Some of the lakes are extremely alkaline (pH ~ 11) and at this pH it might be expected that there would be enhancement of copper due to precipitation. However, there seems no relationship between high copper and high pH suggesting that organic matter content is a more important factor for accumulation of metal.

Moss Mat Sampling Moss mat sampling is often more feasible in high energy, mountain streams where the bed load is depleted in fine sand and silt. Moss also has been found to preferentially capture heavy minerals such as gold and thus improve geochemical anomaly contrast. The ideal sample material will be live moss, growing on boulders or pebbles just above the water level and containing abundant sediment with the moss is handled (gritty to feel). The Red Dog porphyry Cu-Mo-Au-Ag sub economic deposit at the north end of Vancouver Island illustrates the marked difference between the behavior of gold and

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copper in stream and moss sediments. The copper and iron sulphides are hosted in Jurassic Bonanza volcanics. The creek has a 150 m vertical drop over the 3.7 km interval sampled.

The gold profile shows a subdued pattern along the stream reach in sediment, but significant increase at about 3 km down stream from the mineralization. By contrast, the sediment copper profile is similar to the moss. Clearly the gold is being dispersed by a clastic mechanism whereas more hydromorphic processes may transport the copper.

Water Sampling and Analysis Stream or lake water pH and conductivity are measured in the field (in earlier surveys only pH was measured by the lab and usually several months after collection). An unfiltered, un-acidified sample is taken for anion analysis. In some surveys a second sample is taken for determination of trace metals.

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Shown here are 250ml, 125 ml, 60 ml Nalgene sample bottles. The raw water for anion analysis is collected in the 250 ml bottle. A second sample of the raw water for trace metal analysis is collected in the 125 ml bottle and filtered as soon as possible after collection (typically the same day in a field preparation laboratory). A 60 ml syringe is used for filtration attached either a sealed unit on-line filter (more simple to use but more expensive) or an on-line filter with a replaceable 0.45 micron filter paper. Filtered water samples are acidified with ultra-pure nitric acid. Batches sent for analysis include filtered water samples, filtered blank water samples, unfiltered blank water samples, travel blank water samples and water standards.

Above is shown the variation of lead and zinc in sediment downstream from the Bear occurrence SEDEX Pb-Zn-Ag-Ba mineralization. Not only do zinc variations correspond

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to water pH changes but there is also a substantial increase in sulphate probably due to weathering sulphides. There is a simple and rapid field sulphate test the can be carried out at the same time as pH.

Heavy Minerals Concentrating minerals from a bulk sample of stream sediment is another way of dealing with high-energy fluvial sites. Criteria for a good site are: • High energy – shoot and pool profile • Bedrock depression, pothole or crevice. • Clasts should be boulders, cobbles, pebbles and the matrix sand and silt. • Ability to reach bedrock and abundance of well-rounded boulder sized clasts. This shows heavy mineral sampling in a high energy NW British Columbia stream with Dr. Peter Friske, Geological Survey of Canada. About 10 to 12 kg of –12 mesh gravel are recovered by wet screening into a 12 litre pail. The pail is lined with 2 heavy-duty PVC bags that are sealed after the sediment has been screened. Moss and conventional sediment are also taken at each site. (photo credit - Brian Grant).

The map on Page 15 shows sediment samples with and elevated arsenic, antimony and mercury at the 90 percentile threshold. These elements were selected because they are typical pathfinder signature for epithermal gold deposits. Anomalous values clearly outline a belt in north western British Columbia that follows the “Golden Triangle” and includes the Eskay Creek Au-Ag mine.

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This map of the area around the Eskay Mine shows the gold distribution in routine RGS stream sediments. The variation in stream sediment gold content is displayed with different colored catchment basins. Basins with more than 21 ppb Au are outlined in red. Also shown on the slide are the number of gold grains in HM concentrates plus the number of “pristine” (i.e. irregular) grains in red. The higher number of “pristine” grains suggests a proximal source to the gold HM anomaly. In addition to the obvious Eskay mine area with up to 209 pristine grains in the HM concentrates other sources for the gold could be the Iskut-Palmiere prospect north east of the 177 total gold gain count. There is another creek draining an area to the south with 30 gold grains of which 6 are pristine. In both of the creeks that original stream sediment gold values are less than 21 ppb.

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Soil Sampling “Dirt bagging” is often used as a somewhat derogatory term for geochemical soil surveys. In practice soil sampling should be carried out with the same care as geological mapping because there can be a significant difference between A, B and C soil horizon chemistry due to the increased re-distribution of elements in the near-surface “critical zone” where the processes of soil formation are most active. Hence, it is quite possible to introduce a misleading geochemical trend by inadvertently taking samples across horizon boundaries. Textural and colour changes down soil profiles much be carefully observed and recorded during sampling because it is often difficult to distinguish the gradational contact between horizons.

When in doubt about which horizon to sample - Do an orientation survey first!!

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Interpretation – A framework The degree of interpreting survey results can range from just checking of the spatial relationship between the high values and obvious sources for metal (e.g. a known mineral prospect) to analysing the data with multivariate statistics. Before starting on any data analysis there are several basic steps that are needed to establish the credibility of the data (credit for notes – Steve Amor). These are:

• Confirming sample preparation and the analytical techniques used to generate the data. Laboratory reports often arrive with only sketch details and there is no indication about size fraction used, the sample size or sample digestion technique fore analysis.

• Briefly reviewing the data to see if there are any obvious contamination

problems. A sharp, high gold value with a decay curve to lower values through a sequence of samples could mean that a sieve was contaminated during the preparation process.

• Checking on the quality of the data. This will involve looking through the digital

and paper copy files to see if, for example, missing values have been recorded as zeros, there are variable detection limits, the convention for reporting undetectable values (typically < or - ) and quality control data not isolated from routine data.

• Reviewing the quality control data – can all of the elements determined be used.

This may be checking on the quality control, data reported by a commercial laboratory plus results of “blind” QC data (field replicates, standard) sent in with the samples for preparation and analysis.

If the data is acceptable then an analysis can be carried out that could involve the following stages:

• Determining provisional thresholds for each element using basic statistics and/or cumulative frequency histogram. Commonly the 95 percentile value is used as a threshold but a 3rd quartile value can also be used if the data is a combination of several populations.

• Checking on element associations in the raw data, using a correlation matrix and

with scatter plots. Plotting provisional anomaly maps using symbol plots based on either percentile or median-quartile intervals.

• Comparing the symbols plots to a bedrock, surficial geology and landform maps.

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• Plotting element association maps based on the expected signature for different depot types (e.g. As-Sb-Hg for epithermal gold deposits). One limitation of this approach is that a different threshold may be needed for each element to compensate for varying geochemical mobility.

• Ranking anomalies in order of priority for follow-up.

General References Fletcher, W.K. (1997): Stream sediment geochemistry in today’s exploration world. In

Proceeding of exploration 97: Fourth Decennial International Conference on Mineral exploration, Gubins, A.G., Editor, pages 249-260.

Levson, A.A. (1980): Introduction to exploration geochemistry. 2nd Edition. Applied

Publishing. 924 pages. Rose, A.W., Hawkes, H.E., and Webb, J.S. (1979): Geochemistry in mineral exploration.

2nd Edition, Academic Press. Fletcher, W.K., Hoffman, S.J. Mehrtens, M.B. Sinclair, A.J. and Thomson, I. (1987):

Exploration Geochemistry: Design and interpretation of Soil Surveys, SEG Review in Economic Geology 3, 180 pages.